The overall objective of the proposed research is to understand how the normal heart maintains an uninterrupted supply of energy to support its contractile function. This research will investigate the hypothesis that the flow of energy from the mitochondria to the myofibrils is critically dependent upon an adequate supply of creatine phosphate and that the creatine phosphate cannot be replaced by ATP from any source. If this hypothesis is shown to be true, it has major ramifications for all aspects of cardiac contractile metabolism. This research will use, as a primary model, hyperpermeable cardiac bundles. In this preparation, the intracellular organelles retain their integrity but the environment surrounding them can be regulated by varying the concentration in the external solution. The hyperpermeable bundle preparation is exceptionally stable and force measurements are reproducible from contracture to contracture through several activation cycles. This means that even small differences in force production are significant. The specific aims of this proposal include: 1) to measure the total creatine kinase activity in the hyperpermeable bundle; 2) to determine the location of the creatine kinase in detergent treated hyperpermeable bundles by histochemical techniques and determine the relationship between the location and function; 3) to determine the efficiency of the endogenous creatine kinase and how the efficiency depends on the adenine nucleotide concentration; 4) to evaluate the ability of the mitochondria of hyperpermeable bundles to support contractile activity if given a substrate and oxygen; 5) to investigate the possibility that there is a small pool of creatine dedicated to the transport of energy from the mitochondria to the myofibrils; and 6) to estimate the size and turnover rate of the ADP pool associated with myofibrils. The calcium activated force, velocity of shortening and oxygen consumption will be correlated with the functional abilities of the endogenous creatine kinase. This relationship will define the importance of the shuttle to cardiac cells that are maximally stimulated by calcium as well as to resting tissue.